The present invention relates generally to communication systems and, more particularly, to a system and method that use asymmetric codes for spreading and despreading transmissions in a communication system.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing next generation technology.
A current trend in the industry is to move from Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) techniques to Code Division Multiple Access (CDMA) techniques. CDMA is a type of spread spectrum technique that allows signals from multiple users to overlap in both time and frequency by assigning different spreading codes to different users. More specifically, CDMA systems xe2x80x9cspreadxe2x80x9d signals across a common communication channel by multiplying each signal with a unique spreading code sequence, such as a unique orthogonal code. A transmitter then scrambles and transmits the signal on the common channel in overlapping fashion as a composite signal. Each receiver correlates the composite signal with a respective unique despreading code sequence that is an exact replica of the spreading code sequence used to spread the signal at the transmitter to extract the signal addressed to it.
The correlation operation performed at the receiver generally involves chip-by-chip, or even worse, sample-by-sample multiplication of the incoming signal with the value of each chip of the matching spreading code sequence followed by an addition of all the product results. Receivers use matched filters to perform the correlation operation. The multiplication operation performed by the matched filters can range from sign comparisons (i.e., a multiplication by xe2x80x9c1xe2x80x9d or xe2x80x9cxe2x88x921xe2x80x9d) in the case of bi-phase signals, to exclusive-OR (XOR) operations in the case of digital signals, to complex multiplication operations in the case of systems with complex-valued spreading code sequences. As such, receivers often need complex matching filters to despread signals received using spread spectrum techniques. Moreover, by despreading a received signal with a despreading code sequence that is an exact replica of the code used to spread the signal in the transmitter, the receiver may experience increased interference. This increased interference may be due to interference caused by one""s own transmission (e.g., due to multipath) or by the transmission of others (e.g., due to the near-far effect).
There exists a need for a system and method that reduces the interference in spread spectrum transmissions and the complexity of spread spectrum receivers.
Systems and methods consistent with the present invention address this need by providing a mechanism that reduces the interference of spread spectrum transmissions and allows for the implementation of a less complex receiver.
A system, consistent with the present invention, for processing a signal in a wireless network includes a transmitter and a receiver. The transmitter selects a spreading code from spreading matrix, spreads a signal using the spreading code, and transmits the spread signal through the wireless network. The receiver selects a despreading code from a despreading matrix that is the inverse of the spreading matrix, despreads the transmitted signal, and processes the despread signal.
In another implementation consistent with the present invention, a method includes receiving a transmitted signal and generating an orthogonal despreading code that is different than the code used to spread the transmitted signal. The method further includes despreading, the received signal using the orthogonal despreading code and processing the despread signal.
In yet another implementation consistent with the present invention, a method includes generating a spreading code matrix, such that rows of the spreading code matrix correspond to different spreading codes. The method assigns a first group of spreading codes from the spreading code matrix to a first signal to be transmitted and a second different group of spreading codes to a second signal to be transmitted. The method assigns the second different group of spreading codes such that a third group of unassigned spreading codes is located between the assigned first group and the assigned second group of spreading codes. The method spreads the first and second signals using the assigned spreading codes. The method then transmits the first and second spread signals.